An automotive-type solid electrolyte exhaust gas oxygen sensor is disclosed in U.S. Pat. No 3,844,920 Burgett et al. The sensing element in such a sensor is a tapered thimble of zirconia. The interior and exterior of the thimble have separate porous electrode coatings of platinum or the like. The inner electrode is exposed to a source of oxygen, such as air or mixed oxides for establishing a reference potential. It is generally formed by painting a coating of platinum ink onto the thimble, drying it and then firing the coated thimble. An improved technique for applying the coating is described in United States patent application Ser. No. 3,030,776 (now abandoned), entitled "Reference Electrode Process for Exhaust Gas Oxygen Sensor", prepared for filing in the name of John Trevorrow, and assigned to the assignee of this invention.
The outer electrode is exposed to the exhaust gas for establishing a potential determined by its oxygen concentration. This electrode can be a thick film platinum electrode applied in the same manner as the inner electrode. However, it may be more desirable that it be a thin film electrode, applied by evaporation, sputtering, chemical vapor deposition, or other such thin film deposition techniques. However, it has been difficult to consistently reproduce desirable properties in such thin film electrodes. Apparently, thin film electrodes are produced with varying porosity, surface area, platinum microstructure, and/or electrical properties. In any event, it has been difficult to consistently reproduce the most desirable electrical, electrochemical and catalytic parameters in sensors having thin film exhaust electrodes. As a result, yields of the most desirable sensors have been reduced. Various procedures have evolved to improve electrode, i.e. sensor, performance. For example U.S. Pat. No. 3,978,006 Topp et al asserts that nonporous thin film electrodes can be made porous by a heat treatment. U.S. Pat. No. 4,136,000 Davis et al discloses chemical and electrochemical electrode activation treatments. It is also known that sensor characteristics may improve somewhat during initial periods of use. Hence, sensors have also been functional in representative exhaust gas systems before installation in operational systems, to both stabilize and improve sensor properties.
An improved sputtering process for producing platinum thin films is described and claimed in concurrently filed United States patent application Ser. No. 3,030,775 (now abandoned), entitled "Exhaust Electrode Process for Exhaust Gas Oxygen Sensor", filed in the names of Terry J. Gold and Ralph V. Wilhelm, Jr. and assigned to the assignee hereof. The thin films are porous as deposited and have more consistent electrode characteristics. Hence, sensors with more reproducible characteristics are provided. Lean-to-rich switching response times below 600 milliseconds at 700.degree. F. are consistently obtained without using any electrode or sensor post-electroding treatments. Rich-to-lean switching response times may vary from 250 milliseconds to 1200 milliseconds at 700.degree. F. However, they consistently drop to below 600 milliseconds after functional operation for 0.5-3 hours in exhaust gas. We have now found that sensors having such electrodes need not be functionally operated in exhaust gas to reduce their rich-to-lean switching response times and to stabilize them at this low switching time. We have found that an equivalent effect can be obtained by a simple furnace treatment of the zirconia element after electroding and before its incorporation in a sensor assembly.